The RD-114 like feline endogenous virus (FEV) is able to productively infect all of more than 20 embryonic feline cells tested. However, the efficiency of infection of cells from different embryos is markedly different varying up to 100-fold. Studies with Moloney sarcoma virus (SMV) serve to divide the embryonic feline lines into two classes, those which transform ("permissive") and those which do not ("restrictive") when only FEV helper virus is provided. However, both "permissive" and "restrictive" cells allow efficient transformation by the FEV pseudotype of MSV when a heterologous helper virus is added. FEV is a complex of viruses. Two isolates derived from the same FEV stock were found to share considerable genetic homology but had unrelated coat antigens. MSV-m1 infected S+L- lines were developed from various "permissive" and "restrictive" embryonic cells. One of these was an efficient assay line for numerous cat, mouse, and primate C-type viruses. Additionally, the "restrictive" cell lines provided unique properties which will allow further exploration of the mechanism of cell transformation by m1MSV.